Abstract
A footprint algorithm, based on a Lagrangian stochastic (LS) model embedded into a parallelized large-eddy simulation (LES) model, is used for the evaluation of flux and concentration footprints of passive scalars in flow in and above an urban-like canopy layer of a neutrally stratified \(440 \hbox { m}\) deep boundary layer. The urban-like canopy layer is realized by an aligned array of cuboids whose height H is \(40\hbox { m}\). The canopy flow involves strong small-scale inhomogeneities although it is homogeneous at the large scale. The source height is \(1\hbox { m}\) (0.025H) above the ground in the street canyons, roughly mimicking traffic emissions. Footprints are evaluated for four heights from 0.25H to 2.5H, and for up to eight different horizontal sensor positions per measurement height, comprising sensor positions inside as well as outside of the street canyon that extend perpendicular to the mean wind direction. The LES-LS footprints are compared with footprints estimated by a conventional model (Kormann and Meixner, in Boundary-Layer Meteorol 99:207–224, 2001). It becomes evident that the local heterogeneity of the flow has a considerable impact on flux and concentration footprints. As expected, footprints for measurements within and right above the canopy layer show complex and completely different footprint shapes compared to the ellipsoidal shape obtained from conventional footprint models that assume horizontal homogeneity of the turbulent flow as well as the sources of passive scalars. Our results show the importance of street-canyon flow and turbulence for the vertical mixing of scalar concentration.
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The code can be accessed under http://palm.muk.uni-hannover.de/browser.
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Acknowledgments
This study was supported by the German Science Foundation (DFG) under grants RA 617/13-1, RA 617/23-1, and by the German Academic Exchange Service (DAAD), by the Finnish Centre of Excellence in Atmospheric Science – From Molecular and Biological Processes to the Global Climate (project 272041), EU project InGOS Integrated non-CO2 greenhouse gas observation system and ICOS 271878, ICOS-Finland 281255 and ICOS-ERIC 281250 and grant number 138328 funded by the Academy of Finland, and by the European Research Council funded project “Atmospheric planetary boundary layers: Physics, modelling and role in the earth system” (PBL-PMES) (Grant agreement number 227915) and by the CityClim project funded by the Academy of Finland (Grant number 277664). The final computations have been performed on the Cray XC-40 supercomputer of the Center of Scientific Computing (CSC Oy) in Espoo, Finland. CSC is warmly acknowledged for providing us the necessary parallel computing capacity. Preparatory simulations were partly carried out on the IBM Regatta P690 series of the Norddeutscher Verbund für Hoch- und Hochstleistungsrechnen (HLRN) in Hannover/Berlin, Germany and partly on Sun Fire x4600 Cluster Tsubame at the Global Scientific Information and Computing Center of the Tokyo Institute of Technology. Mr. Jin Zhang is acknowledged for modifying the particle boundary condition algorithm of the PALM model for non-flat topographies. Three anonymous reviewers helped improve the manuscript and provided different viewpoints to the problem.
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Hellsten, A., Luukkonen, SM., Steinfeld, G. et al. Footprint Evaluation for Flux and Concentration Measurements for an Urban-Like Canopy with Coupled Lagrangian Stochastic and Large-Eddy Simulation Models. Boundary-Layer Meteorol 157, 191–217 (2015). https://doi.org/10.1007/s10546-015-0062-4
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DOI: https://doi.org/10.1007/s10546-015-0062-4